Family-based treatment (FBT) is a behavioral weight control intervention that targets children who are overweight/obese and their parents, and has the capacity to improve the weight status of non-targeted family members such as siblings. FBT has significant positive effects on body weight in children for up to 10-years of follow-up, and a robust relationship is observed between child and parent outcomes. FBT's concurrent care of two generations of obesity in the family is more efficacious and cost-effective than if family members are treated by their separate health care providers. Despite its recognized efficacy, FBT is mainly available in specialty clinics and many children fail to receive this guideline-based level of treatment, as recommended by the U.S. Preventive Services Task Force. Primary care offers an optimal setting for delivery of FBT by capitalizing on the established relationship between primary care providers and families. Using interventionists co-located within the primary care setting overcomes barriers posed by fragmentation of care, and lack of provider time and training. One of the challenges to integrating childhood obesity treatment into primary care is optimizing limited health care resources. In behavioral weight loss programs, some individuals learn diet, physical activity, and behavior change information quickly, while others learn more slowly. Individuals also differ in their ability to implement treatment recommendations due to individual differences, such as problems with delaying gratification. FBT accommodates these individual differences by using a personalized system of instruction, or a mastery model, in which the content and dose of treatment is calibrated to the needs of the family, ensuring that treatment effort is consistent with need. This multi-site, clinical trial aims to evaluate over a to year period the effectiveness of FBT delivered by a trained interventionist co-located within primary care plus enhanced usual care delivered by the primary care provider (FBT+EUC) compared to enhanced usual care alone (EUC). Participants will be a representative sample of 528 families with a 6-12 year-old child and a parent who are both overweight/obese. Weight changes in approximately 228 siblings who are overweight/obese and between 2-18 years of age will also be studied. This study will test between group differences in child (Primary Specific Aim 1A) and parent (Primary Specific Aim 1B) weight change, as well as weight change in siblings who are overweight/obese (Secondary Aim 1), changes in parent and child delay of gratification, and how changes in delay of gratification are related to parent and child weight changes (Secondary Aim 2), participant level predictors of treatment success (Secondary Aim 3), and how provider attitudes toward evidence-based treatment and perceptions of FBT may relate to their intention to use co- located FBT in their practices in the future (Exploratory Aim 1). Establishing that FBT can be effectively implemented within real world settings is crucial to creating a system by which children and their families who suffer from obesity can be treated in a centralized primary care setting.